This invention arose from an effort to utilize free piston Stirling engine technology for conversion of solar energy to electrical power. The free piston engine produces high pressure hydraulic flow which powers a commercial hydraulic motor that in turn drives a commercial rotary induction generator. The engine development required a high confidence design for a free piston Stirling engine system. Design goals included long projected engine life, high efficiency, minimum life cycle cost, dynamic balancing, minimum weight, heat pipe or reflux boiler thermal transport, a self-contained cooling system, and automatic control.
The approach used to achieve these objectives employed a hermetically sealed Stirling hydraulic engine concept. The basic elements of such engines and their critical components have demonstrated long operating times in supporting experimental work. The described engine concept provides full film hydrodynamic lubrication of all sliding parts, simple construction with conventional automotive manufacturing tolerances, hydraulically coupled counterbalancing, and simple, but effective, power control to optimally follow variations in solar heat input. A novel stabilizer disciplines the reciprocating movement of the displacer to provide control and stabilization functions not previously achieved in free piston Stirling machines.
In the described example for use with solar energy as the heat source for the engine, a suitable concentrator (parabolic reflector) reflects insolation for absorption in a receiver, where it is transferred to the Stirling engine heater head by a simple, rugged and reliable liquid metal pool boiler. Pulsatile hydraulic flow from the Stirling hydraulic engine is smoothed by a pulsation suppressor for use by a hydraulic motor. Engine waste heat is rejected to the atmosphere by a heat exchanger. A simple, energy conservative, automatic control system can be used to adjust engine power to maintain constant hot end temperature in the Stirling engine over a wide range of insolation power levels. The output shaft of a variable displacement hydraulic motor can be coupled directly to an induction generator for producing properly conditioned electrical power. The heat exchanger, surge suppressor, hydraulic motor, and induction generator are all proven, reliable, commercially available components.